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Scientists Slow the Speed of Light
lightbox32 sends news that scientists have found a way to slow individual photons within a beam of light. Their work was published today in Science Express (abstract, pre-print). The researchers liken a light beam to a team of cyclists — while the group as a whole moves at a constant speed, individual riders may occasionally drop back or move forward. They decided to focus on the individual photons, rather than measuring the beam as a whole. The researchers imposed a particular pattern on a photon, then raced it against another photon, and found that the two arrived at their destination at slightly different times.
The work demonstrates that, after passing the light beam through a mask, photons move more slowly through space. Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material—it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.
But, what are the applications of this?
It stayed slow after re-entering the normal environment though.
Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material--it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.
This is not through a medium. Light travels slowly through water, for example, but returns to the speed of light after exiting the water. This is passing photons through a mask, and then those photons staying at the slower speed.
This is incredibly cool. Previous work has managed to fully stop light, but this is quite a finding (that light can travel slower through a vacuum).
The old stuff, from Wiki:
In 1998, Danish physicist Lene Vestergaard Hau led a combined team from Harvard University and the Rowland Institute for Science which succeeded in slowing a beam of light to about 17 meters per second,[1] and researchers at UC Berkeley slowed the speed of light traveling through a semiconductor to 9.7 kilometers per second in 2004. Hau later succeeded in stopping light completely, and developed methods by which it can be stopped and later restarted.
However, now we can alter the structure of the beam of light and measure a slowdown (from the abstract):
Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount.
Details from the pre-print:
We use an ultraviolet laser incident upon a beta-barium borate (BBO) crystal to produce photon pairs with central wavelength at 710 nm. The photons, called signal and idler, pass through an interference filter of spectral bandwidth 10 nm and are collected by polarization-maintaining, single-mode fibers. One fiber is mounted on an axial translation stage to control the path length (Fig. 2A). The idler photon goes through polarization maintaining fibers before being fed to the input port of a fiber-coupled beam splitter (Fig. 2B) (17). Instead of going straight to the other beam splitter input, the signal photon is propagated through a free-space section (Fig. 2C). This consists of fiber-coupling optics to collimate the light and two spatial light modulators (SLMs). SLMs are pixelated, liquid-crystal devices that can be encoded to act as diffractive optical elements implementing axicons, lenses and similar optical components. The first SLM can be programmed to act as a simple diffraction grating such that the light remains collimated in the intervening space, or programmed to act as an element to structure the beam (e.g. axicons or lenses with focal length ). The second SLM, placed at a distance 2, reverses this structuring so that the light can be coupled back into the single-mode fiber that feeds to the other input port of the beam splitter. The output ports of the fiber-coupled beam splitter are connected to single-photon detectors, which in turn feed a gated counter (Fig. 2D). The coincident count rate is then recorded as a function of path difference between the signal and idler arms. The position of the HOM dip is recorded as a function of the spatial shaping of the signal photon.
Surely this wasn't intended behavior? The more we poke at reality, the more it seems like a simulation that works really well, but where you can see some artifacts once you get in close.
Let's not stir that bag of worms...
Not a physicist, but a cyclist and an engineer--
If the population travels as 'c' on average, and they have proven that some photons slow down... Doesn't that mean other photons MUST be traveling faster than c? My impression is the relativity has no bearing here--by traveling at 'c' they are already breaking that equation. The peloton works because some move back while others move up. This blurb seems to only discuss the "back" part.
No, this is progress! One way to travel at the speed of light is to slow down that speed.
My read on this (probably wrong) is that c is the max speed limit and the slow photons are the stragglers of the peloton, which would mean c' for this beam is slightly less then c. Again, probably totally wrong.
"Who are you?" "No one of consequence." "I must know." "Get used to disappointment."
Clearly they didn't slow the speed of light, but sped up time. The speed of light is a constant, the flow of time is not.
"Have you ever thought about just turning off the TV, sitting down with your kids, and hitting them?"
no, c is the top speed of your paceman - in fact, c is the only speed of your paceman. Every other rider can only travel at the same speed or *slower*. Switching pacemen means that your current paceman must drop back (ie slow down) rather than the column speeding up to overtake (thus breaking c). The average speed of the entire column must necessarily be less than c at all times, the guy at the front (doesn't matter who it is) is always the fastest man on the field unless he is dropping back to let the column overtake him - without the column having to speed up.
In cycling, the pace rider may travel at a certain speed (let's call it 40km/h), that may be the designated pace for the event. His replacement may do a short burst at 41km/h to assume the pace position. This breaks the model.
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
I read TFA and could not specifically find where they showed they adjusted the speed and not just added an initial delay. They ran it through a mask, then onto a ~1 meter long "race track" to compare. I really wanted a clear explanation that they ran the test over 2 lengths to factor out any static delay caused by the propagation through the mask itself.
May be, but only for a very short period of time, otherwise the average speed would be increasing or decreasing. So in average they might all move at c/n (in medium of index n) but on a very short time scale they might go slightly faster, or slightly slower, just not always faster or slower...
Somehow it reminds me about US educational system.
Yes, despite what the other person below said. Or above, who knows in these funny Slashdot times.
But before you get your knickers in a twist, there isn't anything useful here to use.
Lets go with the car analogy.
A ferrari is peering its head around a corner as you sit waiting at the traffic lights.
You see only the start of it and automatically assume that it is a ferrari. Who wouldn't? They are pretty unique in design.
Suddenly, out of nowhere, a train flies around the corner at insane speeds instead.
This FTL part of the wave is merely structural, it is the start of group, the start of a wave and nothing more.
There is no known useful information that can be gained from it other than what is expected to happen, but you cannot ever be 100% sure whether it is a ferrari coming around the corner or a train with the front of a ferrari glued to it for fun.
This is called the phase vs group velocity arguments and is often cited as possible FTL communication.
Who knows, maybe one day we will figure something out, this might even help, but presently there is no known way to make heads or tails of it.
With a prism from my telescope and a magnifying glass.
Yeah, for a second I thought "space drive!" and then "wait a minute, surely you'd want to *increase* the speed of light". Serves me right for reading Slashdot before coffee.
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
Umm... This sounds like Physcis 101... Something traveling through a medium vs a vacuum will always be slower was one of the first lessons I learned
Hence "speed of light in a vacuum".
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
I don't think you are seeing the hidden implications of this report.
They are tracking individual photons, implying they know the location of those particles.
But at the same time, they are also keeping tabs on the SPEED of those photons at the same time.
Now the Uncertainty Principle argues against that ever happening, except that's what the researchers* claim. Obviously these guys have invented the Heisenberg Compensator which - as we all know - is a key component to Star Trek teleportation devices. It's just a matter of time now until we will be able to teleport to Alpha Centauri.
* well, that's what the summary of a science news article claims, anyway. I'm so sure it is 100% accurate I didn't even bother to RTFA.
... but isn't Netfilx already doing something like this?
If it moves, tax it. If it keeps moving, regulate it.
By slowing light down, the government will be able to tax and regulate light, dramatically decreasing budget deficits and changing the economic landscape! Of course people with solar panels will be assessed charges based on the amount of light they're using unlike the rest of us who use good old coal fired electricity. Light will now be regulated into special light speed and slower than light speed lanes on the highways with of course, toll booths.
Stop taxing and regulating light now!
Harrison's Postulate - "For every action there is an equal and opposite criticism"
Well I did Read The Fine Article... in what must be the worst ever case of Casting Physics Pearls Before Illiterate Swine.
But I did get this from the abstract, and it summarizes the point of their results:
"Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves."
Pretending this is my office full of bitter coworkers..
The US Senate is planning to vote on changing the speed of light. The old speed was a "hoax" . . .
Schroedinger's Brexit: The UK is both in and out of the EU at the same time!
While this is an interesting read, a lot of the above comments talks about this as it is a general slow down of light. It is not. A light beam emerging from a flashlight still has the same velocity as always. Light travelling in a straight line isn't affected. Only light on a curve is affected.
Underholdning.info
If they indeed can do this, I would have like to have seen a demonstrate interference pattern showing the beat note between the normal beam and the "slowed" beam. It should be roughly as simply as using a beam splitter, one though their mask, then back into a beam combiner. If coherent laser light is pump in the slower photons should create an interference pattern along the length of the beam that any crummy detector should be able to pick up.
Instead they compared time of arrival over a single distance (as best I can tell from TFA), which is subject to systematic offsets, such as the fixed delay to get through the mask.
If some light can be slowed down, and other light can go faster then it, does that mean something can go faster then the speed of light? Einstien disproved?
"National Security is the chief cause of national insecurity." - Celine's First Law
Umm... This sounds like Physcis 101... Something traveling through a medium vs a vacuum will always be slower was one of the first lessons I learned
That fact may be basic physics, but understanding why requires something more.
Possibly the filter that altered the photon that kept a slower speed, may have created virtual viscosity around it that slowed the photon down as if it was traveling though a different medium. Otherwise, it should have sped up to full speed after leaving the filter, even with a changed waveform.
As in does the speed stay the same for a photon traveling 13 billion years? (Is space/time expansion real?)
Photons travel the speed of light unless slowed by a medium. Since there was no medium involved, is what being observed still a photon or instead a photon like particle? Second, it would seem that conservation of mass/energy would indicate if this is a photon then something else must have changed. If there has been some other change, whether we detected it or not, would that not negate the experiment because of a state change (yes, the photon is going slower, but the system is not in the same state it was before)?
If they stay at the slower speed, do they gain mass? I'm not really an expert, but seem to remember that sub-lightspeed particles have mass while particles traveling at the speed of light don't. Feel free to bring me up to... err... speed.
Yeah it's like taking 2 runners of equal speed then putting a small fence on the track in front of one of them and being surprised when the guy with the fence had a slower average speed.
At least you didn't write "Serves me right for reading coffee before drinking my morning Slashdot."
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... do you not understand?
Which might have happened.
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
Smells like a **detector effect**, like photo-electric emission ... a quantized absorber making a continuous wave appear as a "particle". The honest Scotsmen induced distortion of the continuous wavefront thus making the "front-edge" non-detectable ... like tickling a rhinoz *zz with a thrush-feather not a hemp cannon-swab.
refraction do not lower the speed, it increase the distance.
Not a physicist, but a cyclist and an engineer--
If the population travels as 'c' on average, and they have proven that some photons slow down... Doesn't that mean other photons MUST be traveling faster than c? My impression is the relativity has no bearing here--by traveling at 'c' they are already breaking that equation. The peloton works because some move back while others move up. This blurb seems to only discuss the "back" part.
Try reading about phase and group velocities. In fact some EM waves have velocities above c, but these can't convey information so aren't a problem for relativity. This article has a decent discussion of it and other things that go faster-than-light.
"For every expert, there is an equal and opposite expert"
I'm curious on the other side of the equation: (Quote from summary... why RTFA?!) "The researchers liken a light beam to a team of cyclists — while the group as a whole moves at a constant speed, individual riders may occasionally drop back or move forward." Dropping back is fine BUT moving forward should be theoretically impossible since that photon would then be traveling above the speed of light SO are they making the claim that individual photons are able to accomplish such a feat?
Inquiring minds want to know...
Farnsworth: Of course not. That's why scientists increased the speed of light in 2208.
lose != loose
So um, is the speed of light 299,792,458 m / s - (minus) infinity? Meaning, at C, both the creation and discretion of the universe is instantaneous. C, being a reference for all things happening/happened simultaneously?. Thus, Time being an illusion in the disparity between the two??
Life is not for the lazy.
no, because c is a fixed and finite value. It would represent omnipresence and omniscience if c were infinite, but it's not. What it means is that in our frame of reference, where time is a rail which we have absolutely no conscious control over, the further away an event from the observer the longer it takes the light from that event to reach the observer. If the speed of light were infinite, we'd be burned, blind and dying from the sheer pressure of radiation hitting us from all directions.
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
So, they didn't redshift the photon, they made it slower with the same wavelength?
Yes, I meant rest mass, not relativistic mass. But if they are still traveling at light speed but in a roundabout way, like you say, that probably explains it.
Are they sure? Maybe they just lengthened the meter :P
c is ~299,997km/s. It also is a theoretical quantity. If you ever accelerated a mass to c, you'd discover its velocity fluctuates subtly between some miniscule quantity around c. I wonder if we could somehow use this to generate a tachyon?
You read, but you did not comprehend. Yes, their experimental setup accounted for this.
The short form is that they played with the wave. Think of it as giving a (planar) sine wave a sideways tug. At any given moment it's still traveling at c, but it's taking a curvier path to get there.
“Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the
light by a readily measurable amount.”
How they made their light fly in curlicues is definitely interesting, but as for the results, I can only say: Go home, photons, you're drunk.
A photon IS light.
If it's moving faster than the others it's still the speed of light.
Just, you know, this light and not that one.
I ask because we thought that light traveled faster one way than the other once, and it turned out to be a measurement error in the circuitry.
Could the low temp also slow down the reporting devices? Things behave strangely when the temperature drops too much.
-- Tigger warning: This post may contain tiggers! --
At least you didn't write "Serves me right for reading coffee before drinking my morning Slashdot."
thanks, I was wondering why it tasted so bad